This application claims the benefit of a Japanese Patent Application No. 2004-325145 filed Nov. 9, 2004, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to magnetic recording media and magnetic storage apparatuses, and more particularly to a magnetic recording medium employing the in-plane or longitudinal magnetic recording technique and to a magnetic storage apparatus which uses such a magnetic recording medium.
2. Description of the Related Art
Magnetic storage apparatuses, such as magnetic disk drives, are popularly used to store digital image data related to dynamic images, audio data related to music and the like. The magnetic storage apparatus is particularly used increasingly as a video recorder for home use, to replace the video tape recorder for home use, due to the improved performance of the magnetic storage apparatus, such as the capability to perform a high-speed access, the compact size of the apparatus and the large storage capacity of the apparatus. Hence, the market for the magnetic storage apparatus is rapidly increasing. The amount of information (or information quantity) included in the dynamic image data (or video data) is extremely large, and the magnetic disk drive needs to have a large capacity in order to store the dynamic image data. For this reason, the recording density of the magnetic recording media have increased at a rate of almost 100% per year, and it is essential to develop techniques for the magnetic recording medium and the magnetic head that would enable further increase in the recording density of the magnetic recording medium.
In order to realize a high recording density on the magnetic recording medium, that is, in order to record information in a recording layer of the magnetic recording medium with a high density, it is necessary to reduce the area of a minimum recording unit that is magnetically formed on the recording layer in correspondence with one bit of information. The isolated reproduced waveform half-width, namely W50, is used as an index for indicating the reduction of the area of the minimum recording unit. The W50 is the half-width of one peak (reproduced waveform) that is obtained by making a recording on the recording layer of the magnetic recording medium by reversing a recording magnetic field only once and then reproducing the same. In general, the recording and reproducing resolution becomes higher as the W50 becomes larger, and the magnetic recording medium having a small W50 is suited for realizing the high recording density.
The W50 is described by 2{(g/2)2+(d+e)2}1/2, where g denotes a gap length of a reproducing element of the magnetic head, d denotes a magnetic spacing between the recording layer and the reproducing element of the magnetic head, and e denotes a magnetic transition width of the recording layer. It may be seen that one method of reducing the W50 is to reduce the magnetic transition width e of the recording layer. A magnetic transition region is formed between a region that is magnetized in one direction by the recording and a following region (magnetization region) that is magnetized in the reverse direction with respect to the one direction. In other words, the magnetic transition region is necessary to reverse the magnetization direction. The magnetic transition width e is the length of the magnetic transition region along the recording direction.
Normally, a ferromagnetic CoCr alloy is used for the recording layer of the magnetic recording medium. Hence, the recording layer is a polycrystal made up of a large number of crystal grains of the CoCr alloy. In the magnetic transition region, it is known that the magnetic transition width e decreases as the individual crystal grains become smaller, and studies have been made to reduce the individual crystal grains in order to decrease the magnetic transition width e.
However, when the size of the crystal grains of the recording layer is excessively reduced, a phenomenon in which the magnetization of the magnetization layer decreases with the lapse of time becomes conspicuous. This phenomenon occurs due to the following. That is, the barrier height of the energy barrier for reversing the magnetization of the individual crystal grains forming the magnetization region decreases as the crystal grains become smaller. In addition, a sum of the temperature (thermal energy) and the energy of the reverse magnetization field generated in the reverse direction to the magnetization becomes larger than the barrier height, to thereby reverse the magnetization of the crystal grains. Hence, this phenomenon is caused by the deterioration of the thermal stability.
KuV/kT is used as an index for indicating the thermal stability, where Ku denotes an uniaxial anisotropy constant indicating the magnitude of the anisotropic energy of the recording layer in the direction of the axis of easy magnetization, V denotes a volume of the crystal grains of the recording layer, k denotes a Boltzmann's constant, and T denotes the temperature of the magnetic recording medium. Hence, KuV/kT is represented by a ratio of the energy KuV that indicates the extent to which the magnetization direction is fixed and the thermal energy kT that causes instability of the magnetization direction. The thermal stability is better when the value of KuV/kT is larger.
Conventionally, studies have been made on the composition of the recording layer for the purposes of increasing the value of KuV/kT. For example, Pt was added to the CoCr alloy or the CoCrTa alloy forming the recording layer, so as to increase the uniaxial anisotropy constant Ku and consequently increase the value of KuV/kT.
On the other hand, studies have also been made on the so-called mechanical texturing which mechanically forms a large number of grooves in the recording direction on the substrate surface or the like. Due to the internal stress that is induced by the undulations of the mechanical texturing, the axes of easy magnetization of the individual crystal grains of the recording layer become oriented in the recording direction. As a result, it is possible to increase the uniaxial anisotropy constant Ku and consequently increase the value of KuV/kT.
The applicant is aware of a Japanese Laid-Open Patent Application No. 2002-260210 which proposes a magnetic recording medium taking measures to improve the thermal stability.
However, if the Pt content of the recording layer is excessively increased, the crystallinity of the parent phase of CoCr deteriorates, and as a result, the thermal stability deteriorates and the medium noise increases. In addition, the anisotropic field which indicates the magnitude of the recording field necessary to rotate the magnetization increases, to thereby deteriorate the recording characteristics such as the overwrite performance.
On the other hand, there is a limit to improving the coercivity in the recording direction solely by the mechanical texturing. In other words, if the depth of the grooves of the mechanical texturing is increased to increase the coercivity, the undulations of the mechanical texturing increases. Since the undulations are inherited to the surface of the magnetic recording medium, the increased undulations would increase the surface roughness of the magnetic recording medium. In such a magnetic recording medium having the increased surface roughness, it becomes difficult to reduce a distance (floating distance) between the magnetic head and the surface of the magnetic recording medium. As a result, it becomes difficult to reduce the W50 which depends on the magnetic spacing d between the recording layer and the reproducing element of the magnetic head.